Social disparities in mental and physical health are among the most universal, enduring, and costly problems of human societies. Despite pervasive, graded health effects at every level of the socioeconomic scale, little is understood of the mechanisms by which such effects occur or how human development influences their character and magnitude. The health consequences of SES may be due not to differentials in living conditions, lifestyle or health care, but rather to experiences of social ordering per se, that is, the subjective human experience of occupying a lesser or greater position on a scale of social influence. If so, children's experiences with social ordering in early childhood groups may represent a first, formative encounter with the hierarchical social relations affecting health over the human lifespan. To investigate these possibilities, the researchers will: 1) study predictive associations between social ordering and children's mental and physical health during the kindergarten and first grade years; 2) examine relations among social processes-including social ordering, peer rejection, and victimization-and their relative contributions to health; and 3) investigate antecedents of social position among background child factors, family sociocultural factors, and baseline health status. A prospective, multiple cohort design will be employed, in which 300 kindergarten children, 100 each in three academic year cohorts, will be followed from the August preceding kindergarten entry through the end of first grade. Social position will be ascertained using repeated naturalistic observations, a measure of the child's subjective social status, and an experimental protocol designed to elicit hierarchical behavior. Other peer processes (peer rejection and victimization) will be ascertained with peer nomination, child interviews, and parent and teacher reports. Antecedent independent variables (child's gender, birth order, physical size and strength, temperament, family SES and sociocultural values) will be evaluated with direct measurement, psychometric instruments and parent questionnaires. Finally, outcome measures-comprising indicators of mental and physical health-will be assessed with a combination of standardized stress reactivity procedures, parent- and teacher-report questionnaires, child self-report protocols, and frequent, objective physical examinations of the child.